Processes for simulating corrosion product deposits (sludge) are now well known. In one known method, alumina silicate binders are combined with dry powder mixtures of metal oxides, mineral species and metals, such as copper or zinc, in order to form coatings of synthetic deposits. The synthetic deposits based on alumino silicate binders have been previously used for chemical and mechanical cleaning qualification programs. The known process may be used to simulate corrosion product depositions on both flat and curved surfaces. However, these alumino silicate based deposits lack sufficient consolidation, strength and water resistance for many heat transfer test purposes. Consequently, there is a need for a process of forming an adherent metal oxide coating that can withstand severe flow boiling conditions up to about 200,000 BTU/hr.multidot.ft.sup.2 heat flux. One such process described herein is based on the use of an orthophosphoric acid binder with mixed metallic oxide and metal powders.
The use of orthophosphoric acid for consolidating some oxides is briefly discussed in ASM Handbook Volume 5, Surface Engineering (page 472, published by ASM international, copyright 1994). In the ASM Handbook, the oxides used are limited to aluminum oxide, zinc oxide, hafnium oxide, and zirconium oxide.
U.S. Pat. No. 5,620,645 discloses mixing metal or metal compounds with orthophosphoric acid and then adding a moldable filler material, usually refractory in nature. Preferred materials include aluminum hydroxide, chromium trioxide, magnesium oxide, and zinc oxide, The mixture is placed in a mold and dried at a temperature of less than 100.degree. C. to remove water and harden the material, The material is then oven dried at 300.degree. C. where additional water is driven off and the mixture is pressed into its final configuration and desired porosity. The liberation of H.sub.2 gas and the resulting increase in porosity caused by the reaction of orthophosphoric acid and metal species is either not desired or useless to the method described in this patent. This patent also makes no direct or indirect reference to simulation of heat-exchanger deposits.
U.S. Pat. No. 1,761,186 discloses dipping, repeatedly, if desired a ferrous material heated to 500.degree. F. (260.degree. C.) in an orthophosphoric acid solution to apply a relatively thin, rust inhibitive coating of ferrous-ferric oxide directly on the ferrous material. The coating is then oven dried. It is contemplated that dissolving manganese, zinc or iron phosphate in the acid could result in these rust inhibiting compounds forming part of the coating. Porous coatings are not desired for such an application. The use of a phosphoric acid solution to rust-proof steel surfaces is, according to the description in this patent, an established part of the prior art. However, the stated purpose of the described process is to create a phosphatic, rust-inhibiting, coating.
U.S. Pat. No. 4,643,778 is similar to U.S. Pat. No. 1,761,186 in that a thin coating is chemically formed directly on the base metal by dipping the material in a solution of phosphoric acid In this case, the film or coating formed allows adherence of paint. The coating formed is either zinc phosphate or iron phosphate. The relevant properties of the zinc phosphate or iron phosphate coatings that result from the process described in the patent concern only the corrosion protection provided.